Generally, excavating and earth moving operation by use of a work vehicle such as a bulldozer is often carried out with the vehicle engine being in full operation. In such operation, when the drive of the vehicle is switched from a forward speed range to a reverse speed range or vice versa, or when the operation of the vehicle is suspended for a while with the engine running, fully operated engine generates extremely high noise and causes high fuel consumption. To cope with such situations, the operator depresses the decelerator pedal or turns the fuel throttle dial to its low idle side, thereby reducing engine speed. However, if the operator depresses the decelerator pedal or turns the fuel throttle dial in every vehicle suspension, it will impose serious burden on the operator.
To solve this problem, there have been proposed various techniques for reducing engine speed by actuating an automatic decelerator when work implement levers are brought into their neutral position to stop the work implements.
For instance, Japanese Patent Publication (KOKOKU) Gazette No. 5-52411 (1993) discloses a technique wherein engine speed is slightly reduced by actuating the automatic decelerator for a short time, upon placement of all the work implement levers in their respective neutral positions and after the engine is maintained at this reduced speed for a specified time, the automatic decelerator is again operated to further reduce engine speed. Another technique is disclosed in Japanese Patent Publication (KOKOKU) Gazette No. 60-38561 (1985) according to which, when a low speed drive control command has been issued, a low engine speed drive is instructed after an elapse of a predetermined delay time after all the operation levers are placed in their neutral positions, and measurement of the delay time is cleared at the time when at least one of the operation levers is actuated before an elapse of the delay time. Japanese Patent Publication (KOKOKU) Gazette No. 61-34327 (1986) discloses a system which is arranged such that a supply of fuel to the internal combustion engine is stopped provided that the running condition of the internal combustion engine meets a specified level, and if the gear position of the transmission is changed from its neutral position to a non-neutral position, an interruption in fuel supply will be inhibited to prevent engine stall.
The conventional techniques disclosed in these publications, however, suffer from the problem that when operation is resumed after suspension with the engine running at a reduced speed and engine speed is restored to an initial speed, occurrence of an instantaneous impact load is unavoidable at the time of engagement of the gear shift clutch of the transmission.
The present invention is directed to overcoming the foregoing problems and the prime object of the invention is, therefore, to provide an engine speed control system for a work vehicle which reduces engine speed when operation is suspended for a while in order to reduce engine noise and suppress fuel consumption, the system enabling a reduction in a possible instantaneous impact load which would occur at the time of engagement of the gear shit clutch when operation is resumed after suspension.